The present invention pertains to remotely reconfigurable antennas, and particularly to reconfiguring antennas by optical control of mechanical switches.
Reconfigurable antenna systems have applications in satellite and airborne communication node (ACN) systems where wide bandwidth is important and where the antenna aperture must be continually reconfigured for various functions. These antenna systems may comprise an array of individually reconfigurable antenna elements. Each antenna element may be individually reconfigurable to modify its resonant frequency, such as by varying the effective length of dipole elements. Varying the resonant frequency of individual elements may enable an antenna to operate at a variety of frequencies, and may also enable control of its directionality.
One means of varying the resonant length of a dipole antenna is to segment the antenna lengthwise on either side of its feed point. The resonant length of the antenna may then be varied by connecting or disconnecting successive pairs of adjacent dipole segments. Connection of a pair of adjacent dipole segments may be effected by coupling each segment to a switch. The adjacent segments are then joined by closing the switch.
Previous designs for reconfigurable antennas have been proposed which incorporate photoconductive switches as an integral part of an antenna element in an antenna array. See xe2x80x9cOptoelectronically Reconfigurable Monopole Antenna,xe2x80x9d J. L. Freeman, B. J. Lamberty, and G. S. Andrews, Electronics Letters, Vol. 28, No. 16, Jul. 30, 1992, pp. 1502-1503. Also, the possible use of photovoltaic activated switches in reconfigurable antennas has been explored. See C. K. Sun, R. Nguyen, C. T. Chang, and D. J. Albares, xe2x80x9cPhotovoltaic-FET For Optoelectronic RF/Microwave Switching,xe2x80x9d IEEE Trans. On Microwave Theory Tech., Vol. 44, No. 10, October 1996, pp. 1747-1750. One problem with these designs, however, is that the performance of ultra-broadband systems (i.e., systems having an operating frequency range of approximately 0-40 GHz) utilizing these types of switches suffer in terms of insertion loss and electrical isolation.
RF MEMS (micro-electromechanical) switches have been proven to operate over the 0-40 GHz frequency range. Representative examples of this type of switch are disclosed in Yao, U.S. Pat. No. 5,578,976; Larson, U.S. Pat. No. 5,121,089; and Loo et al., U.S. Pat. No. 6,046,659. Previous designs for reconfigurable antennas using RF MEMS switches incorporated metal feed structures to apply an actuation voltage from the edge of a substrate to the RF MEMS switch bias pads. A problem with the use of metal feed structures to apply an actuation voltage to the switches is that, in an antenna array, the number of switches can grow to thousands, requiring a complex network of bias lines routed all around the switches. These bias lines can couple to the antenna radiation field and degrade the radiation pattern of the antenna array. Even when the bias lines are hidden behind a metallic ground plane, radiation pattern and bandwidth degradation can occur unless the feed lines and substrate feedthrough via conductors are very carefully designed because each element in the antenna array may accommodate tens of switches. This problem is magnified enormously as the number of reconfigurable elements increases.
A conductive ground plane generally provides a phase shift of 180xc2x0 upon reflection of electromagnetic waves. In practice, the conductive ground plane should be separated from the antenna elements by at least a quarter wavelength, to avoid destructive interference at the antenna elements between electromagnetic waves received directly at the antenna elements and waves received via reflection from the ground plane. Hence, if the switches are disposed above a conductive ground plane, the bias lines for the switches will extend at least one quarter wavelength above the ground plane. Bias lines of this length above the ground plane may provide the radiation pattern and bandwidth degradation described above.
Thus, there exists a need for a means to control selectable RF MEMS switches in an array to control antenna elements, while reducing interference from control lines.
The present invention solves the above-noted problem by providing a mechanism for optical control of an array of MEM switches which in turn modify antenna elements.
MEM switches are mounted on an antenna substrate so as to provide selectable connections between adjacent elements of an antenna structure. The switches are optically controlled, preferably by means of an active LED matrix or an LCD matrix. Control is preferably provided through a structure adjacent to the antenna array, which shields the optical control circuitry and preferably provides a reflective surface to aid the antenna. The low-power, voltage-controlled MEM switches are provided with an actuating bias voltage, either by means of direct connections, through the reflective surface if used, or by means of an illuminated series of photovoltaic (PV) cells. Optical control of each MEM switch is preferably provided by a photoresistive element that shunts the bias source to deactuate the switch.
The preferred reflective surface presents a high impedance to electromagnetic waves in the antenna operating frequency range, and accordingly reflects the waves with little or no phase shift (less than 90 degrees, and preferably near 0). This reduces array-to-reflector spacing distance and alleviates bandwidth constraints, which are imposed by that spacing. The preferred embodiment of the present invention includes a high impedance reflective surface fabricated on a multilayer printed circuit board as a matrix of conductive pads, each having controlled capacitance to adjacent pads and having a via with controlled inductance connecting from its center to a common plane on the opposite side of the board. The controlled inductance vias, or other vias through the reflective surface, may provide for light transmission from the active matrix optical panel to the photoelectric elements controlling the MEM switches, and may also conduct bias voltage for the switches. The antenna array elements are preferably disposed on a substrate positioned above the front side of the high-impedance surface of the circuit board and much less than xc2xc wavelength from the front side of the high-impedance reflective surface.